The present disclosure relates to an optical fiber coupler for coupling pump radiation into an optical fiber, and a method for manufacturing the optical fiber coupler.
Gain media based on optical fibers (e.g., fiber lasers and amplifiers) are finding increasing applications in advanced laser sensor systems. These gain media based on optical fibers (e.g., fiber lasers and amplifiers) have a broad range of performance features including high efficiency, robust single-mode output, high reliability, compact coiled packaging, large surface-area-to-volume ratio for favorable thermal performance, and an all-fiber architecture without any free-space optics (i.e., no requirement for a rigid optical bench). Over the past decade, the output powers of fiber lasers have increased by several orders of magnitude, for example, from the watt-level to multi-kW powers.
U.S. Pat. No. 5,864,644, the entirety of which is hereby incorporated by reference, describes a Tapered Fiber Bundle (TFB) for coupling light into and out of cladding-pumped fiber devices. According to this architecture, a group of multi-mode pump fibers are combined with a single-mode signal fiber to form a close-packed bundle. This close-packed bundle is then heated and stretched to produce a tapered bundle with a final diameter approximately equal to the diameter of one of the original fibers. For example, a bundle of six pump fibers and one signal fiber (e.g., each having a diameter of 125 μm) has an initial cross-sectional dimension of 375 μm (i.e., 3×125 μm). When heated and tapered, the final TFB will have a diameter of 125 μm, and will contain the original power in all of the input fibers with insertion losses kept within a few percent. The TFB is then fusion-spliced to form a double-clad, or cladding pumped, gain fiber in which all of the pump power efficiently enters the pump cladding, where it will ultimately be absorbed by the active doped core. The signal beam also efficiently passes through the TFB into the core of the gain fiber. TFBs are widely recognized as one of the enabling technologies supporting development of high-power fiber lasers and amplifiers. In all known types of TFBs, the pump and signal beams propagate through independent circular cores that provide index-based guiding in all transverse directions. The tapered architecture may generate optical power losses, and these losses may pose thermal loading challenges for high pump powers.
U.S. Pat. No. 6,477,295, the entirety of which is hereby incorporated by reference, describes a bulk pump-coupling block of glass that is bonded to a fiber for which the outer cladding is removed. Pump light is then directed into the block in a manner such that it will enter the gain fiber at the boundary of the block with the gain fiber. This reference envisions a free-space coupling. Thus, this design does not provide an all-fiber monolithic design. This reference also requires that the fiber core be continuous and uninterrupted throughout the coupler, making the fabrication of the pump-coupling block very challenging, and requiring a significant amount of handling of raw, uncoated fiber, which might pose a possibility of contaminating the fiber surface. Also, any pump power that has not been absorbed (i.e., by the time it propagates to the next pump coupler down the fiber length) will be lost unless additional modifications are made to the fiber. Further, the design disclosed in this reference also requires precise matching of the indices.
U.S. Pat. No. 6,529,318, the entirety of which is hereby incorporated by reference, describes a prism-coupling approach that is conceivably scalable to relatively high powers. This reference describes several approaches whereby prisms attached to flat surfaces on a gain fiber are used to reflect pump beams into a gain fiber. This reference only considers prisms that direct free-space pump beams into the gain fiber. Thus, this reference does not provide an all-fiber monolithic design. The architecture disclosed in this reference is inherently single-sided (i.e., one cannot inject pump light from two opposite sides of the fiber at the same location), thereby reducing the possible pump power per injection point. Although rectangular claddings are envisioned in this reference, this reference does not disclose a high-aspect ratio rectangular core or a semi-guiding core. A high-aspect ratio rectangular core is a core shaped as a rectangle rather than a circle, where one dimension of the rectangle, the slow-axis direction, is much larger (e.g., by a factor of 10 to 100 or more) than the other dimension of the rectangle, the fast-axis direction, and in which index-based guiding is employed at all four core-cladding interfaces. The high-aspect ratio rectangular core is considered in the art to be “semi-guiding” if the index-based guiding is employed in only one transverse direction (i.e., the fast-axis direction). Further, the optical adhesive used to attach the prism to the fiber may lead to reliability problems due to the degradation of the adhesive under strong illumination of the pump source.
What is needed, then, is a means of coupling pump power into the rectangular cross-section core gain fiber that does not involve any free-space optics and/or does not pose thermal loading challenges for high pump powers.